During the refining of metals, such as copper and other non-ferrous metals, the electrolytic process produces a residue known as anode mud or anode slime. The mud (slime) typically contains valuable elements such as gold, silver and selenium. In addition, there are commonly practiced hydrometallurgical extracting processes where the leaching of base metals leads to precious metal bearing residues which can be further treated for gold and silver recovery. These metallurgical residues are in some instances further treatable with acid leaching in the presence of very strong oxidizing agents to recover the valuable metals. This however, inevitably leads to dissolution of nearly all the metals and hence to expensive separation processes. A further complication is that a separate process is required to recover the selenium contained in the residue. Tailings from mineral processes could also bear precious metals which are economically recoverable.